In Otto engines Lambda probes are used to determine the exhaust gas composition in order to comply with the statutory requirements in relation to restricting pollutant emissions. In order to comply with current and future exhaust gas legislation requirements catalytic converter systems close to the engine are increasingly being used to convert the pollutants. Because of the short exhaust gas mixing path between the exhaust valves of the cylinder and the catalytic converter these demand a low tolerance in the fuel-air ratio between the cylinders of an exhaust bank of an internal combustion engine by comparison with catalytic converters arranged away from the engine. A catalytic converter is arranged away from the engine for example if the catalytic converter is mounted under the vehicle floor. For such systems with catalytic converters arranged close to the engine, in which as well as an optimization of the flow formation for mixing the exhaust gases, the said small tolerance in the fuel-air ratio between the cylinders of an exhaust bank of the internal combustion engine is required, cylinder-selective Lambda control is also used in addition to the conventional Lambda control.
In recent times it is precisely the cylinder-selective Lambda control based on the use of piezo injectors that has increased in importance. As well as their many advantages, these types of injector are also characterized by their high tolerance to variations.
The cylinder-selective Lambda control requires as its input signal a measure for the fuel-air ratio of each individual cylinder of the internal combustion engine. Since usually however only one Lambda probe is used for all cylinders of an exhaust bank, the Lambda values for each individual cylinder must be obtained from the signal of this Lambda probe. The prerequisite for detecting the cylinder-selective Lambda variations is for sufficient information (useful signal) to be able to be derived from the probe signal. Since the signal of the linear Lambda probe is overlaid with noise in the sampling frequency band, the amplitude of the useful signal must be much higher than that of the noise signal. Furthermore it must be ensured that, depending on the configuration of the exhaust system and the position of the Lambda probe, the signal quality of the useful signal falls as the exhaust mass flow decreases. The disadvantage emerging from this is that in the lower load range the signal strength of the useful signal disappears in noise. Thus under these conditions a stable control of the exhaust composition the aid of the useful signal cannot be guaranteed.
In order to provide a stable Lambda control the function of cylinder-selective Lambda control was previously switched off in the lower load and speed range of the internal combustion engine. The disadvantage of doing this is that there is no compensation for the cylinder-selective variations of the injection system in the lower load range. Thus the adaptation values which are learned in the mid load and speed range of the internal combustion engine with the aid of the cylinder-selective Lambda control function (ZSLR) are not able to be transferred to the lower load and speed range of the internal combustion engine.